Up to now, an active matrix (hereinafter, referred to as AM) OLED display has been studied as a light-emitting display device in which pixels each including an OLED element and a drive circuit are arranged in matrix. This example is illustrated in FIGS. 8 and 9.
FIGS. 8 and 9 illustrate an internal structure of a pixel of the AM OLED display and a pixel arrangement thereof, respectively. As illustrated in FIG. 8, a pixel 10 includes an OLED and a drive circuit 11 having an active element connected with an anode terminal thereof. The drive circuit 11 is connected with a data line DL and a scanning line SL. This example in the figure illustrates the case where one scanning line SL is provided. As illustrated in FIG. 9, multiple pixels, each of which is the pixel 10 including the OLED and the drive circuit 11, are arranged in matrix (m rows×n columns) and connected with first to m-th scanning lines SL1 to SLm and first to nth data lines DL1 to DLn.
According to the AM OLED display having the structure as described above, a voltage, a current, or the like which is supplied to an OLED element is controlled by an active element of a drive circuit based on a voltage or current signal applied to the drive circuit of a pixel through a data line. Therefore, the luminance of the OLED element is adjusted for gradation display. A thin film transistor (TFT) is normally used as the active element which is a constituent element of the drive circuit.
In the AM OLED display, there is a problem of a temporal change in voltage-luminance characteristic of the OLED element. Also, there are problems in that variations in characteristics of the TFTs and changes in characteristics of the TFT due to electrical stress occur. In the case where the characteristics change or vary as described above, even when the same signal is applied to the drive circuit from the data line, the luminance of the OLED element changes. Therefore, display unevenness, a bright point, a dark point, or the like appears. Thus, in order to realize high-quality displays, it is necessary to develop a drive circuit and a driving method which are resistant to the temporal change in characteristic of the OLED element and the variations and changes in characteristics of the TFT.
In order to solve the problems of the drive circuit, conventional techniques have been proposed in U.S. Pat. No. 6,373,454 and U.S. Pat. No. 6,501,466.
According to U.S. Pat. No. 6,373,454, a current corresponding to light-emitting luminance of an OLED element is supplied from the outside of a pixel to a driver (p-type) TFT for supplying a current to the OLED element to hold a voltage between a gate terminal and a source terminal between which the current flows. Then, the current determined based on the held voltage between the gate terminal and the source terminal is supplied to the OLED element through the TFT, so the OLED element emits light. In this example, the voltage between the gate terminal and the source terminal between which the current corresponding to light-emitting luminance flows is held and the TFT acts as a constant current source. Therefore, even when the characteristics of the driving TFT vary, the current supplied to the OLED element does not vary.
According to U.S. Pat. No. 6,501,466, one of two TFTs forming a current mirror structure is a driver (p-type) TFT for supplying a current to an OLED element and the other thereof is a load (p-type) TFT to which a current corresponding to light-emitting luminance of the OLED element is supplied from the outside of a pixel. The current is supplied from the outside of the pixel to hold a voltage between a gate terminal and a source terminal which corresponds to the current flowing into the load TFT. Then, the current determined based on the held voltage between the gate terminal and the source terminal is supplied from the driving TFT to the OLED element, so the OLED element emits light. Even when the characteristics of the TFTs vary depending on positions, the driving TFT and the load TFT are located close to each other and exhibit the same characteristic, so the current supplied to the OLED element does not vary as in the case of U.S. Pat. No. 6,373,454.
A semiconductor such as polycrystal silicon (hereinafter, referred to as p-Si), amorphous silicon (hereinafter, referred to as a-Si), an organic semiconductor (hereinafter, referred to as OS), or a metal oxide semiconductor has been studied as a material for a channel layer of the TFT.
A p-Si TFT has high mobility, so an operating voltage thereof can be reduced. However, because of crystal grain boundary, variations in characteristics are more likely to increase and a manufacturing cost becomes larger. On the other hand, an a-Si or OS TFT has lower mobility than the p-Si TFT, so the operating voltage is high and thus power consumption is large. However, the number of manufacturing steps is small, so the manufacturing cost can be suppressed. In recent years, a TFT using a metal oxide semiconductor such as zinc oxide (ZnO) for a channel layer has been under development and it has been reported that the TFT may have higher mobility and lower cost than those of the a-Si and OS TFTs.
Unlike the p-Si TFT, it is difficult to use the a-Si, OS, or metal oxide semiconductor TFT for a complementary TFT in which an n-type TFT and a p-type TFT are formed on the same substrate. For example, in the case of a-Si or metal oxide, a high-mobility p-type semiconductor is not obtained, so it is difficult to form the p-type TFT. In the case of OS, because a high-mobility n-type semiconductor material is different from a high-mobility p-type semiconductor material, the number of steps is doubled, so low-cost manufacturing is difficult to achieve. Therefore, it is necessary to use only the n-type or p-type TFT for the drive circuit using the TFTs.
In the TFT whose channel layer is made of one of a-Si, OS, and metal oxide, a current-voltage characteristic thereof is changed by the application of a voltage for a long time, so it is necessary to compensate for the change by any method.
On the other hand, the OLED element normally has a structure in which at least a light-emitting layer made of an organic material is sandwiched between an anode electrode and a cathode electrode. It is more likely to change characteristics of the organic material by the influence of heat, an electromagnetic wave, or moisture. Therefore, a manufacturing process for forming the organic material light-emitting layer after the formation of the drive circuit and the anode electrode and then forming the cathode electrode by vacuum vapor deposition with less damage is preferably used for a light-emitting display device using the OLED element.
Then, assume that a pixel of the AM OLED display includes the drive circuit having the n-type TFT and the OLED element having the anode electrode, the organic light-emitting layer, and the cathode electrode which are formed in the stated order from the lower side. In such a case, the display cannot be realized by only replacing the p-type TFT of the drive circuit described in U.S. Pat. No. 6,373,454 or 6,501,466 with the n-type TFT. This is because, when the p-type TFT is replaced with the n-type TFT in U.S. Pat. No. 6,373,454 or U.S. Pat. No. 6,501,466, a voltage between the gate terminal and a drain terminal is fixed, so the TFT doe not act as the constant current source. Therefore, it is necessary to employ a drive circuit structure different from that in U.S. Pat. No. 6,373,454 or U.S. Pat. No. 6,501,466.
A drive circuit proposed in FIG. 2 of Japanese Patent Application Laid-open No. 2004-093777 includes only n-type TFTs. This is a technique for suppressing the influence of variations in characteristics and the influence of changes in characteristics. The drive circuit includes a capacitor provided between a gate terminal and a source terminal of an n-type TFT (driving TFT) for driving an OLED element. For a period in which a current for driving the OLED element is set, a gate terminal and a drain terminal of a TFT are electrically connected with each other to cut off a path to the OLED element and supply a current from the outside. At this time, a voltage between the gate terminal and a source terminal corresponds to a voltage (set voltage) when the current supplied from the outside flows. For a period in which the OLED element is driven, the n-type TFT acts as a constant current source for supplying the current to the OLED element based on the set voltage.
In recent years, a current-luminance characteristic of the OLED element has been improved to reduce a current supplied to the OLED element. A large-size and high-definition OLED display is required, so it tends to increase a line load. Therefore, when a low current corresponding to low gradation is supplied from the outside in Japanese Patent Application Laid-open No. 2004-093777, a time for charging the line load becomes longer. Thus, it is difficult to apply the drive circuit described in Japanese Patent Application Laid-open No. 2004-093777 to a high-definition and large-screen display device.
For example, assume that a capacitance and a resistance of the line load of a large-screen display device are 40 pF and 5 kΩ (time constant is 0.2 μsec.), respectively, and a variation in voltage which is required to set the current supplied from the outside is 3 V. In this case, the amount of charge to be stored is 120 pC. When the line load is to be charged with a current of 10 nA corresponding to low gradation, a time of 12 msec. is required. When scanning lines (1250) of a high-definition television are to be driven at 60 Hz, a selection period per scanning line is 13 μsec., so charging is impossible.
A means for solving the above-mentioned problems is proposed in FIG. 1 of Japanese Patent Application Laid-open No. 2004-093777. According to the drive circuit, a charging current can be increased up to approximately ten times larger. In such a case, the charging period can be shortened from 12 msec. to 1.2 msec. However, it is insufficient to use the drive circuit for the high-definition television.
Another means for solving the above-mentioned problems is a drive circuit illustrated in FIG. 1 of Japanese Patent Application Laid-open No. 2005-189379. The drive circuit has a function of correcting a threshold voltage of a driving TFT. In the circuit, a current for driving an OLED element is set based on a voltage from the outside. A setting period is mainly determined based on a charging period of a line load. The time constant of the line load is 0.2 μsec. Therefore, when a period during which 99.8% charging is completed is assumed as the setting period, the period becomes 1.2 μsec which is six times the time constant. Therefore, when this conventional technique is used, a high-definition television can be driven.
However, in this circuit, a voltage applied between a gate terminal and a source terminal of the driving TFT is determined based on a divided voltage obtained by two capacitors provided in the drive circuit. Therefore, in order to realize high-precision driving, it is necessary to provide two capacitors in a pixel to realize a precise capacitance ratio between the capacitors.
Another drive circuit for solving the above-mentioned problems is proposed in J. H. Jung et al., SID 05 DIGEST 49.1, FIG. 1. In this circuit, as in the circuit described in Japanese Patent Application Laid-open No. 2005-189379, the current for driving the OLED element is set based on the voltage from the outside, so the setting period can be shortened. In this circuit, the voltage applied to the gate terminal of the driving TFT is determined by only one of the capacitors and the other of the capacitors is used for only storage, with the result that a variation in ratio between the capacitors does not become a problem.
However, in the circuit, the voltage between the gate terminal and the source terminal of the driving TFT is not fixed. The driving TFT operates not as the constant current source but as a source follower for applying a voltage to the source terminal. A voltage obtained by correcting threshold voltages of the driving TFT and the OLED element is applied to the gate terminal of the driving TFT. Therefore, only when a change in voltage-current characteristic of the OLED element is shifted in parallel relative to the applied voltage, this correction is established.